The present invention relates generally to a supercharging apparatus for an internal combustion engine equipped with a mechanical supercharger, and more particularly to a supercharging apparatus for an internal combustion engine which is capable of varying valve closing timing of an in take valve in accordance with an engine operational condition.
There has been conventionally proposed various supercharging systems for an internal combustion engine equipped with a mechanical supercharger. For example, Unexamined Japanese Patent Publication No. 63-239312 discloses an engine control technique improving knocking without reducing intake air charging efficiency. For this end, this prior art discloses a variable valve timing means to largely delay the closing timing of an intake valve or enlarge an overlap between the intake and exhaust valves.
This technique is excellent in view of bringing numerous advantages in various controls of an engine equipped with a supercharger. However, there are some aspects to be further improved. For example, when the closing timing of the intake valve is largely delayed, the mechanical supercharger tends to receive an increased resistance especially in a low engine speed region. Because intake air once introduced into the cylinder is partly fed back toward the intake pad, sage as the intake valve opens late until the compression stroke progresses.
For this reason an increased resistance or load is imparted on the mechanical supercharger in this engine operational condition. Although the power of the supercharger may be forcibly increased, the resistance or load will be correspondingly increased. This apparently results in worse fuel economy.
The Unexamined Japanese Patent Publication No. 2-119624 discloses another engine control technique wherein a maximum supercharging pressure from the supercharger is changeable in accordance with an engine speed. In order to reduce the shock occurring in the change of the maximum supercharging pressure, this prior art proposes to cooperatively utilize dynamic supercharging effect so as to cancel or moderate this shock. However, the teaching of this latter prior art cannot be directly used for solving the above-described problem of the former prior art since the problem inherently relates to the delayed valve closing timing of the intake valve.